1. Field of the Invention
The present invention relates to an ignition coil, and particularly to the structure of a secondary bobbin which constitutes the ignition coil.
2. Discussion of Background
In the past, ignition coils for an internal combustion engine have been constructed as shown in FIGS. 5 and 6. In FIGS. 5 and 6, the ignition coils comprise a casing 1 made of a thermoplastic material, an iron core 2 inserted in a cylindrical hole la which is arranged in the center of the casing 1 as one unit, and a primary coil 3 placed around the outer wall of the cylindrical hole 1a. The primary coil is constituted by winding a primary winding 3b on a primary bobbin 3a in 50-150 turns. The primary winding 3b has a wind-starting end 3c and a wind-ending end 3d.
Around the primary coil 3 is placed a secondary coil 4. The secondary coil 4 comprises a secondary bobbin 4a and a secondary winding 4b wound on the secondary bobbin in about 6,000-12,000 turns. A high voltage cord 5 has one end inserted in a recess 6 which is formed to extend from the bottom of the casing 1 inwardly. Into the end surface of the one end of the cord is pierced a high voltage terminal 7 like a rivet from the inside of the casing 1 through a top wall of the recess. The secondary bobbin 4a of the secondary coil 4 has a top circumferential end with a binding portion 4c formed integrally with the circumferential end so that the binding portion projects outwardly in the radial direction of the bobbin. The binding portion 4c is in the form of T-letter as seen from above the bobbin. The secondary winding 4b has a one end connected to the high voltage terminal 7 by soldering or similar way after a portion of the secondary winding close to the connected end is wound around the binding portion 4c in several turns. The other end of the secondary winding 4b is connected to the wind-ending end of the primary winding 3b of the primary coil 3. A thermosetting resin 8 such as epoxy resin is filled in the casing 1 in order to fix the primary coil 3 and the secondary coil 4 in an insulating state in the casing 1.
In the conventional ignition coils, the secondary winding 4b which is wound on and around the seconary bobbin 4a has the portion close to the connected end wound on and around the T-shaped binding portion which is formed the top circumferential edge of the bobbin 4a, and has the connected end connected to the high voltage terminal 7. It is a disadvantage of the conventional ignition coils that the wire wound on the binding portion can lift, or sometimes comes loose due to slack to form a short circuit with the secondary winding. If such short circuit is formed, a required voltage can not be obtained with the secondary coil 4 and the discharge voltage at a spark plug in the internal combustion engine is thus decreased to cause a flame-out.